A successor communication system to W-CDMA and HSDPA, i.e., Long Term Evolution (LTE), is currently being discussed by 3GPP that is a standardization group for W-CDMA. In LTE, orthogonal frequency division multiplexing (OFDM) is to be used as a downlink radio access method and single-carrier frequency division multiple access (SC-FDMA) is to be used as an uplink radio access method (see, for example, 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006).
In OFDM, a frequency band is divided into multiple narrow frequency bands (subcarriers) and data are transmitted on the subcarriers. The subcarriers are densely arranged along the frequency axis such that they partly overlap each other but do not interfere with each other. This approach enables high-speed transmission and improves frequency efficiency.
In SC-FDMA, a frequency band is divided into multiple frequency bands and the frequency bands are allocated to different terminals for transmission in order to reduce interference between the terminals. Also, SC-FDMA reduces variation of the transmission power and therefore makes it possible to reduce power consumption of terminals and to achieve wide coverage.
In uplink and downlink of LTE, one or more physical channels are shared by multiple mobile stations for communication. A channel shared by multiple mobile stations is generally called a shared channel. In LTE, a physical uplink shared channel (PUSCH) is used for uplink and a physical downlink shared channel (PDSCH) is used for downlink.
In a communication system employing shared channels, it is necessary to report (or signal) allocation information of the shared channels to mobile stations for each subframe (1 ms in LTE). In LTE, a control channel used to report the allocation information (signaling) is called a physical downlink control channel or a downlink (DL) L1/L2 control channel. The physical downlink control channel, for example, includes a downlink (DL) L1/L2 control format indicator, downlink (DL) scheduling information, acknowledgement information (ACK/NACK), an uplink (UL) scheduling grant, an overload indicator, and a transmission power control command bit (see, for example, R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding). The DL L1/L2 control format indicator is also called a physical control format indicator channel (PCFICH) and the ACK/NACK is also called a physical hybrid ARQ indicator channel (PHICH). The PCFICH and the PHICH may be defined as independent physical channels instead of as components of the PDCCH. The DL scheduling information may also be called a downlink scheduling grant or downlink assignment information.
The DL scheduling information and the UL scheduling grant are used to report (signal) allocation of shared channels to mobile stations. The DL scheduling information, for example, includes downlink resource block allocation information, UE IDs, the number of streams, information regarding precoding vectors, data sizes, modulation schemes, and information regarding hybrid automatic repeat request (HARQ) for a downlink shared channel. The UL scheduling grant, for example, includes uplink resource block allocation information, UE IDs, data sizes, modulation schemes, uplink transmission power information, and information regarding a demodulation reference signal used in uplink MIMO for an uplink shared channel.
Below, an exemplary uplink communication process using a shared channel is described.
In uplink, the base station selects mobile stations allowed to communicate using a shared channel in each subframe (1 ms) and requests the selected mobile stations via the uplink scheduling grant to communicate using the shared channel. The selected mobile stations in turn transmit the shared channel based on the uplink scheduling grant. Then, the base station receives and decodes the shared channel transmitted from the mobile stations. In the above communication process, selecting mobile stations (user devices) allowed to communicate using the shared channel is called “scheduling”.
In the process, the base station cannot directly determine the amount of data stored in a buffer of each mobile station which are to be transmitted using an uplink shared channel. Therefore, for example, each mobile station transmits a buffer status report indicating the status of the buffer (buffer status) of the mobile station to the base station. In HSUPA, for example, the mobile station reports UE buffer occupancy (in bytes) to the base station. The UE buffer occupancy is a signal for reporting a buffer status of the mobile station, i.e., the amount of a signal in a buffer of the mobile station (see, for example, 3GPP TS 25.309 (V6.6.0), “FDD Enhanced Uplink, Overall description, Stage 2”, 9.3.1.1.1, 2006-03; and 3GPP TS 25.321 (V6.8.0), “Medium Access Control (MAC) protocol specification”, 9.2.5.3.2, 2006-03).